The present invention relates to a mask used for fabrication of a microlens used as an imaging device or an image sensor, a fabrication method for a microlens using the mask, and an imaging device.
A fabrication method for a microlens according to the related art will be described with reference to FIGS. 11A to 11E and FIG. 12. A fabrication method for a microlens according to the related art is also disclosed in Japanese Unexamined Patent Publication No. Hei 4(1992)-12568.
First, a photosensitive microlens material 20 is coated onto a substrate 1 (FIG. 11A). Next, the microlens material 20 is irradiated with a light beam 41 (for example, I-rays) through a mask 30 used for obtaining a desired lens (FIG. 11B). The mask 30 used here has such a layout that a light-blocking region 31 (black portion) is arrayed as illustrated in FIG. 12. The microlens material 20 is irradiated with the light beam 41 through the light-blocking region 31.
Next, a photosensitive portion 21 is removed by developing the microlens material 20 with a developer, thereby forming a pattern 22 that serves as a base from which to fabricate a lens (FIG. 11C). The pattern 22 illustrated in FIG. 11C is a pattern to which the light-blocking region 31 of the mask 30 has been transferred.
Next, the entire surface of the substrate 1 on which the pattern 22 has been formed is irradiated with a light beam 42 (for example, a light beam including I-rays) (FIG. 11D). Through this treatment, the pattern 22 decreases in heat resistance due to reaction of the photosensitive groups in the pattern. Next, by applying heat treatment (baking) to the pattern 22, heat sag (a phenomenon in which a material softens, and changes into a shape as given by the balance between gravity and surface tension in accordance with the level of the softening) occurs, resulting in the shape of a microlens 24 (FIG. 11E). As the heat-sagged microlens 24 is cooled, a plurality of individual microlenses are fabricated on the substrate.
When used in an imaging device, for example, each microlens obtained in this way is required to allow external light to be efficiently captured on the photodiode in the imaging device. To that end, it is desired to minimize the space (narrow the spacing) between individual lenses on the substrate.
With the fabrication method according to the related art as described above, by taking into account the fact that the spacing between the individual patterns of microlens material obtained by irradiation with a patterned light beam through the mask (patterned exposure) becomes narrower due to heat sag, the spacing of each individual light-blocking region of the mask is set to the narrowest possible spacing that does not result in contact between individual microlenses, thereby obtaining a microlens with little redundant space. In the case where a microlens is fabricated by this method, the minimum spacing that allows individual microlenses to be resolved (separated) with good reproducibility depends on the precision with which the extent of spreading of the microlens material upon heat sag can be controlled.
However, with the method according to the related art, to control the extent of spreading of the microlens material upon heat sag with high precision, it is necessary to perform precise control of the temperature profile and temperature distribution during heat treatment, the uniformity of the composition of the microlens material, and so on. There is a limit to implementing such control in the actual production, and also the required process control is extremely complicated. Therefore, there is a limit as to how narrow the microlens spacing can be made by the method according to the related art. In actuality, although depending on the wavelength of the light beam used or the kind of the microlens material, when the spacing between the light-blocking regions of the mask becomes about 0.5 μm, a phenomenon occurs in which the spacing between the individual microlenses obtained becomes sharply narrow, causing bridging between the individual microlenses.
With the fabrication method according to the related art, to make the distance between individual microlenses narrow, it is necessary to make the material to spread widely due to heat sag at the substrate interface. Thus, the height of a microlens obtained due to heat sag becomes small. This results in the shape of a microlens having a very small contact angle with the underlying substrate as illustrated in FIG. 13A, or the shape of a microlens as illustrated in FIG. 13B, and it is difficult to obtain a microlens with an ideal shape as illustrated in FIG. 13C that provides a large contact angle and allows a sufficient height to be kept. It should be noted that as the shape of a microlens, it is desired to make the contact angle large to secure a sufficient height for the microlens itself, in other words, to make the radius of curvature small, because such a shape allows a wider range of light to be collected.
Japanese Unexamined Patent Publication No. Hei 10(1998)-74927 discloses a fabrication method for a microlens which can make the radius of curvature of a microlens larger, and make the radius of curvature the same between the transverse and longitudinal directions of the lens having a rectangular shape, by using a mask for microlens pattern. The mask for microlens pattern has a main light-blocking region, and an auxiliary light-blocking region formed in the vicinity of the main light-blocking region in such a way that its light transmittance increases with increasing distance from the main light-blocking region (see claim 1, and FIGS. 7, 9, and the like of Japanese Unexamined Patent Publication No. Hei 10(1998)-74927). However, the mask used in such a fabrication method has a problem in that fabrication of such a mask required high cost, because the resulting layout of the mask becomes extremely complicated, and also accurate designing becomes necessary. Moreover, the fabrication method disclosed in Japanese Unexamined Patent Publication No. Hei 10(1998)-74927 is not aimed at reducing the microlens spacing.